Heat oven to 180°C (fan forced) and line a 20 x 30cm cake tin with baking paper.Put the apple in a small pan with 2 tbsp water. Cook for a few mins or until the apple starts to soften.Meanwhile, make the crumble topping. Rub the butter into the flour, sugar and lemon zest until it resembles big breadcrumbs, stir in the oats and then set aside.Beat the butter and sugar in a large bowl until fluffy, then gradually add the egg. Tip in the flour and milk, and continue to beat until everything is combined.Incorporate the apples then spoon the mixture into the tin, smooth the surface, then dot with the raspberries.Sprinkle over the crumble topping and bake for 45 mins or until a skewer inserted comes out clean and the topping is golden.